Editing Biotite (section)

==Properties==

Like other [[mica]] minerals, biotite has a highly perfect [[basal cleavage]], and consists of flexible sheets, or [[Lamellae (materials)|lamellae]], which easily flake off.  It has a [[monoclinic crystal system]], with [[tabular habit|tabular]] to [[Prism (geometry)|prismatic]] crystals with an obvious [[wikt:pinacoid|pinacoid]] termination.  It has four prism faces and two pinacoid faces to form a [[Hexagonal (crystal system)|pseudohexagonal]] crystal.  Although not easily seen because of the cleavage and sheets, fracture is uneven. It appears greenish to brown or black, and even yellow when [[weathering|weathered]]. It can be transparent to opaque, has a vitreous to pearly [[Lustre (mineralogy)|luster]], and a grey-white [[Streak (mineralogy)|streak]]. When biotite crystals are found in large chunks, they are called "books" because they resemble books with pages of many sheets. The color of biotite is usually black and the mineral has a hardness of 2.5–3 on the [[Mohs scale of mineral hardness]].

Biotite [[Solvation|dissolves]] in both [[acid]] and [[Base (chemistry)|alkaline]] [[aqueous solution]]s, with the highest [[Dissolution (chemistry)|dissolution]] rates at low [[pH]].<ref>{{cite journal|last1=Malmström|first1=Maria|last2=Banwart|first2=Steven|title=Biotite dissolution at 25°C: The pH dependence of dissolution rate and stoichiometry|journal=Geochimica et Cosmochimica Acta|date=July 1997|volume=61|issue=14|pages=2779–2799|doi=10.1016/S0016-7037(97)00093-8|bibcode=1997GeCoA..61.2779M}}</ref> However, biotite dissolution is highly [[Anisotropy|anisotropic]] with crystal edge surfaces ([[Miller index|''h k''0]]) reacting 45 to 132 times faster than basal surfaces ([[Miller index|001]]).<ref>{{cite journal|last1=Hodson|first1=Mark E.|title=Does reactive surface area depend on grain size? Results from pH 3, 25°C far-from-equilibrium flow-through dissolution experiments on anorthite and biotite|journal=Geochimica et Cosmochimica Acta|date=April 2006|volume=70|issue=7|pages=1655–1667|doi=10.1016/j.gca.2006.01.001|bibcode=2006GeCoA..70.1655H}}</ref><ref>{{cite journal|last1=Bray|first1=Andrew W.|last2=Oelkers|first2=Eric H.|last3=Bonneville|first3=Steeve|last4=Wolff-Boenisch|first4=Domenik|last5=Potts|first5=Nicola J.|last6=Fones|first6=Gary|last7=Benning|first7=Liane G.|title=The effect of pH, grain size, and organic ligands on biotite weathering rates|journal=Geochimica et Cosmochimica Acta|date=September 2015|volume=164|pages=127–145|doi=10.1016/j.gca.2015.04.048|bibcode=2015GeCoA.164..127B|doi-access=free|hdl=20.500.11937/44349|hdl-access=free}}</ref>

<gallery widths="180px" heights="120px" >
File:Biotite mica 2 (31739438210).jpg|Flaky biotite sheets.
File:BiotitaEZ.jpg|Thick biotite sample featuring many sheets.
File:Biotite1.jpg|Biotite crystal exhibiting pseudohexagonal shape.
</gallery>

===Optical properties===

In [[thin section]], biotite exhibits moderate [[Optical relief|relief]] and a pale to deep greenish brown or brown color, with moderate to strong [[pleochroism]]. Biotite has a high [[birefringence]] which can be partially masked by its deep intrinsic color.<ref name="min-in-thin">{{cite web|url=http://funnel.sfsu.edu/courses/geol426/Handouts/mintable.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://funnel.sfsu.edu/courses/geol426/Handouts/mintable.pdf |archive-date=2022-10-09 |url-status=live|title=Identification Tables for Common Minerals in Thin Section|last=Faithful|first=John|year=1998|access-date=March 17, 2019}}</ref> Under [[Polarized light microscopy|cross-polarized light]], biotite exhibits extinction approximately parallel to cleavage lines, and can have characteristic [[Bird's eye maple (mineral property)|bird's eye maple extinction]], a mottled appearance caused by the distortion of the mineral's flexible lamellae during grinding of the thin section. Basal sections of biotite in thin section are typically approximately hexagonal in shape and usually appear [[isotropic]] under cross-polarized light.<ref name="min-rock-sec">{{cite book|title=Minerals in Rock Sections: The Practical Methods of Identifying Minerals in Rock Sections with the Microscope|url=https://archive.org/details/mineralsinrocks02luqugoog|quote=bird's eye extinction thin section grinding.|last=Luquer|first=Lea McIlvaine|year=1913|publisher=D. Van Nostrand Company|edition=4|location=New York|page=[https://archive.org/details/mineralsinrocks02luqugoog/page/n111 91]}}</ref>

<gallery widths="180px" heights="120px" >
File:Muscovite and Biotite2a.jpg|Biotite (in brown) and muscovite in an [[orthogneiss]] thin section under plane-polarized light.
File:Thin Section of Biotite (test) (cropped to Biotite).jpg|Biotite in thin section under cross-polarized light.
File:Sagenitic biotite.JPG|Basal section of biotite, with needle-like [[rutile]] inclusions, in thin section under plane-polarized light.
</gallery>

===Structure===
Like other micas, biotite has a crystal structure described as ''TOT-c'', meaning that it is composed of parallel ''TOT'' layers weakly bonded to each other by [[cation]]s (''c''). The ''TOT'' layers in turn consist of two tetrahedral sheets (''T'') strongly bonded to the two faces of a single octahedral sheet (''O''). It is the relatively weak ionic bonding between ''TOT'' layers that gives biotite its perfect basal cleavage.{{sfn|Nesse|2000|p=238}}

The tetrahedral sheets consist of silica tetrahedra, which are silicon ions surrounded by four oxygen ions. In biotite, one in four silicon ions is replaced by an aluminium ion. The tetrahedra each share three of their four oxygen ions with neighboring tetrahedra to produce a hexagonal sheet. The remaining oxygen ion (the ''apical'' oxygen ion) is available to bond with the octahedral sheet.{{sfn|Nesse|2000|p=235}}

The octahedral sheet in biotite is a trioctahedral sheet having the structure of a sheet of the mineral [[brucite]], with magnesium or ferrous iron being the usual cations. Apical oxygens take the place of some of the hydroxyl ions that would be present in a brucite sheet, bonding the tetrahedral sheets tightly to the octahedral sheet.{{sfn|Nesse|2000|pp=235–237}}

Tetrahedral sheets have a strong negative charge, since their bulk composition is AlSi<sub>3</sub>O<sub>10</sub><sup>5-</sup>. The trioctahedral sheet has a positive charge, since its bulk composition is M<sub>3</sub>(OH)<sub>2</sub><sup>4+</sup> (M represents a divalent ion such as ferrous iron or magnesium) The combined TOT layer has a residual negative charge, since its bulk composition is M<sub>3</sub>(AlSi<sub>3</sub>O<sub>10</sub>)(OH)<sub>2</sub><sup>−</sup>. The remaining negative charge of the TOT layer is neutralized by the interlayer potassium ions.{{sfn|Nesse|2000|p=238}}

Because the hexagons in the T and O sheets are slightly different in size, the sheets are slightly distorted when they bond into a TOT layer. This breaks the hexagonal symmetry and reduces it to monoclinic symmetry. However, the original hexahedral symmetry is discernible in the pseudohexagonal character of biotite crystals.

<gallery>
File:Mica T.png|View of tetrahedral sheet structure of biotite. The apical oxygen ions are tinted pink.
File:Mica tO.png|View of trioctahedral sheet structure of biotite. The binding sites for apical oxygen are shown as white spheres. Red spheres are hydroxide ions.
File:Mica tOs.png|View of trioctahedral sheet structure of mica emphasizing magnesium or iron sites
File:Mica tri.png|View of biotite structure looking at surface of a single layer
File:Mica tri side.png|View of biotite structure looking along sheets
</gallery>